In many types of vehicle powered by an engine, the designer has selected a V-belt CVT. Such a transmission or drive comprises a primary variator and a secondary variator. The primary variator is connected to and driven by the crankshaft of the engine. The motion is transmitted by a V-belt to the secondary variator. The secondary variator is in turn connected to an output drive shaft that drives a wheel, belt, track or the like depending on the type of vehicle. Examples of vehicles where the V-belt CVT is used are light motorcycles and cross-country vehicles, such as three-wheel or four-wheel off-road motorcycles, weasels and snowmobiles.
The variator pulleys, on which the V-belt runs, each consist of two pulley halves. The relative distance of the pulley halves is variable so as to allow different gear ratios. Such shifting traditionally occurs automatically and continuously. As a rule the automatic function is established by the primary variator having a pulley with a fixed pulley half, i.e. which is fixedly connected to the crankshaft of the engine, and a movable pulley half, which is axially displaceable. The displacement of the movable pulley half is controlled by what is referred to as a centrifugal clutch with weight arms which, as the pulley rotates, strive to displace the movable pulley half so that the distance between the pulleys decreases. A return spring strives to pull the movable pulley half in the opposite direction. From an initial position with a stationary variator up to an engagement speed where the weight arms act on the movable pulley half by a force that exceeds the spring force, the distance between the pulleys is maximal. By replacing the spring by a spring with another spring constant and changing the design of the weight arms, it is possible to provide different engagement speeds and operating ranges.
The secondary variator also has a fixed pulley half and a movable pulley half. The movable pulley half is movable both axially and tangentially. A biasing spring presses the movable pulley half against the fixed pulley half and tangentially against a stop. A device with wedge-shaped cams is designed so that the movable pulley half must be displaced tangentially, i.e. rotated about the variator shaft to be able to be displaced axially from the fixed pulley half. This device makes the variator sensitive to torque. With no torque, or only a minor torque, on the output shaft of the variator, it is only necessary for the axial and tangential bias of the biasing spring to be overcome for the movable pulley half to be displaced from the fixed pulley half. With greater torques, greater friction is generated between the movable pulley half and the wedge-shaped cams. The increased friction cooperates with the biasing spring to keep the pulley halves together.
The construction of the primary variator with weight arms, that are actuated by the rotation and control the distance between the pulley halves, is self-regulating and besides results in a fixed operating behavior. It is in many cases desirable to be able to easily change the character of the drive.
An attempt to achieve this is disclosed in European Patent Application Publication No. 701 073, Piaggio et al. The object presented in EP 701 073 is to provide a V-belt CVT for a light motorcycle where the V-belt CVT in addition to the continuous operating mode also provides an operating mode where the driver can select a desired ear. This is achieved by an operating linkage which is connected to the movable pulley half of the primary variator and which is operated by a motorized cam element, which in turn is controlled by an electronic control unit. The driver can select manual or automatic shifting. In the former case, the driver himself increases or decreases the transmission ratio by a button on the handle bars. In both cases, it is the control unit that controls via the operating linkage the displacement of the movable pulley half, either according to a predetermined operating schedule or according to the driver's instructions. The initial setting of the primary variator, however, is still performed by means of a centrifugal clutch. In the automatic position, the control unit then controls the variator based on measured values of the speed of the engine and the position of the throttle lever.
The solution shown in EP 701 073 is certainly applicable to light motorcycles that are driven on roads. However, problems arise if you try to apply the solution to vehicles that are driven under more extreme operating conditions, such as three-wheel and four-wheel off-road motorcycles and snowmobiles, i.e. in the typical case cross-country vehicles. Such more extreme operating conditions comprise, for example, rapidly and sharply shifting friction between the drive wheel/drive belt/track and the ground. Under such conditions, the prior-art solution has difficulties in continuously providing optimal gear ratio.